Are you with Unifi or Time? Did you buy the router yourself? If you got the router from Unifi or Time, you should not have updated the firmware as it is for retail unit, not Unifi or Time supplied router.

Try to check your PPPoE username and password, or check if your VLAN tagging is enabled.

retail firmware seem to not support unifi-pppoe (on the 850L with TM logo) for me, i can see it as an option but i can't save it. Every reboot, it keep default back to pppoe.

Assessing the dlink interface with my phone doesn't show unifi-pppoe option at all.

Reflash 2.08TM firmware and everything works again. Gonna wait for TM version of the latest firmware.

i use dlink recovery mode to re-flash 2.08TM, since the usual interface only allows upgrade

i learned from this site : http://forums.dlink.com/index.php?topic=44909.0

preparation :
-download 2.08TM firmware on your computer
-connect your computer to router using LAN cable
-set your computer's ethernet ip to static ip 192.168.0.2

assess recovery mode :
-power off your router
-use a pin ( or something similar) and poke/press the reset button
-while hold-press the reset button for 3-5 seconds, press the power button while hold-press the reset button until you see the power indicator is in amber colour and blinking ( now router is in recovery mode )

re-flash 2.08TM firmware
-on your computer, open up browser and login to 192.168.0.1
-it will show Dlink Recovery mode
-click select to choose the 2.08TM firmware, and then click upload
-wait until the process finish, it will autorestart
-check the version, it should say 2.08TM 

setup unifi and wifi settings
-since the router is reset, all settings are lost
-you will need to re-enter all unifi settings and personal settings again.

It worked for me, i hoped it works for you 

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So nice, my router i reset also can't connect to internet   

Wah? U guys even the settings cannot save?

What browser did u guys use? I use chrome browser and it works fine, I remembered reading somewhere that other browser might cause that save issue...

And, it's best practice after flashing the firmware, reset the router to factory default settings, I encountered no internet also at first but after done that, set everything again, now all good except the things i mentioned b4...